Passive four-channel decoder

ABSTRACT

A stereophonic sound system effectively produces four distinct signal outputs at four points surrounding a listener. The system utilizes conventional stereophonic records, tapes or broadcasting channels and is adapted for use with a two-channel stereophonic broadcast receiver. The preferred embodiment disclosed utilizes 90* encoded signals to form two distinct composite signals each containing three of the four components required for quadraphonic reproduction. In a room orientation having a right front, left front, right back and left back speaker layout, the two front speakers are coupled respectively to a pair of balanced amplifiers which receive the respective composite signals. Also coupled to each balanced amplifier is a symmetrical lattice network for shifting the phase of the composite signal coupled to that particular amplifier. The lattice networks are coupled to a conventional transformer having a center tapped secondary winding, one of the networks being coupled across the transformer&#39;&#39;s primary winding and the other being coupled between the secondary center tap and a common point between the right and left back speakers which form the load on the transformer. The composite signals coupled to the respective balanced amplifiers are shifted in phase by different amounts and combined in the lattice-transformer network to form a pair of resultant signals. The combination apparatus produces electrical and acoustical signal separation, i.e., the resultant signal produced at the right back speaker has virtually no left back component and likewise the left back speaker has virtually no right back component. Thus, the system eliminates cross-talk in the two rear speakers and complements the composite signals supplied to the front speakers in effectively producing four-channel sound.

i United States Patent [191 Schott Nov. 11, 1975 PASSIVE FOUR-CHANNEL DECODER [75] Inventor: Wayne M. Schott, Mt. Prospect, Ill.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

[22] Filed: Jan. 17, 1974 [21] Appl. No.: 434,454

Primary E.\'aminerl(athleen H. Claffy Assistant Examiner-Thomas DAmico Attorney, Agent, or FirnzCornelius J. OConnor [57] ABSTRACT A stereophonic sound system effectively produces four distinct signal outputs at four points surrounding a listener. The system utilizes conventional stereophonic records, tapes or broadcasting channels and is adapted for use with a two-channel stereophonic broadcast receiver. The preferred embodiment disclosed utilizes 90 encoded signals to form two distinct composite signals each containing three of the four components required for quadraphonic reproduction. In a room orientation having a right front. left front. right back and left back speaker layout. the two front speakers are coupled respectively to a pair of balanced amplifiers which receive the respective composite signals. Also coupled to each balanced amplifier is a symmetrical lattice network for shifting the phase of the composite signal coupled to that particular amplifier. The lattice networks are coupled to a conventional transformer having a center tapped secondary winding. one of the networks being coupled across the transformers primary winding and the other being coupled between the secondary center tap and a common point between the right and left back speakers which form the load on the transformer. The composite signals coupled to the respective balanced amplifiers are shifted in phase by different amounts and combined in the lattice-transformer network to form a pair of resultant signals. The combination apparatus produces electrical and acoustical signal separation. i.e.. the resultant signal produced at the right back speaker has virtually no left back component and likewise the left back speaker has virtually no right back component. Thus. the system eliminates cross-talk in the two rear speakers and complements the composite signals supplied to the front speakers in effectively producing four-channel sound.

3 Claims, 10 Drawing Figures 1 IE! I l [RIGHT ICHANNEL l AMPLIFIER |CHANNEL AMPLIFIER US. Patent N0v. 11, 1975 Sheet20f3 3,919,478

|80 LATTICE E w W A L O FREQUENCY f/f0- U.S. Patent Nov. 11, 1975 Sheet 3 013 3,919,478

3 7R E B PASSIVE FOUR-CHANNEL DECODER BACKGROUND OF THE INVENTION Since four-channel sound has been-introduced, many devices for reproducing the four discrete input signals have been introduced to the stereo industry. The reproduction systems available are of three types.

Through the use of active devices in a four channel matrix decoding receiver, a pair of encoded signals are decoded into the original four independent signals and each signal is translated to its respective directional output or speaker. The industry refers to this type of reproduction system as a logic matrix four-channel sound system.

A second system provides apparatus for matrixing sum-and-difference encoded signals and producing matrixed decoded signals for translation to each of four different speakers in an attempt to create an illusion of four-channel sound. Since each composite signal comprises all four input signals, that is, R,-, L R and L mere matrixing of signals does not provide complete separation; it is inevitable that the respective signal outputs at the two rear and two front speakers have crosstalk. An example of cross-talk is where the signal developed at the right back speaker contains a portion of the left back signal along with the right back signal and a portion of the right back signal is developed at the left back speaker along with the left back signal. A similar type of signal mixing occurs at the respective left and right front speakers, using this type of decoding. This type of decoding is undesirable in that it does not provide acceptable acoustical separation because there is only an amplitude or signal strength separation between the right signal components in the right front and back speakers and the left signal components in the left front and back speakers, a condition which produces an undesired acoustical effect on the listeners ears.

There are ways of matrixing theencoded signals to effectively separate the two composite signals into four distinct signals for translation'to four corresponding speakers thereby creating an illusion of four-channel sound. Many of these devices are able to adequately separate the signals being translated to a left front and right front speaker simply because the quadrature encoding apparatus produces two composite signals each respectively comprising only one of the two front signal components available, whereas each composite signal comprises both back signals. This separation of the composite signals into two separate front speakers is accomplished by merely coupling each respective composite signal to its corresponding front speaker. However, since each composite signal comprises both rear signals, to effectively or adequately separate the back signals into their respective back speakers, it has been necessary to use active devices to matrix the signals. These represent an undesirably expensive addition to a consumers present two-channel stereophonic system, when all that is gained is an illusion of four-channel sound and not true discrete four-channel sound.

SUMMARY OF THE INVENTION The present invention proposes to produce, through the use of passive lattice networks, four distinct signals for transmission through four corresponding sound .speakers to effectively create four-channel sound from a pair of quadrature-encoded composite signals. The composite signals contain 'right front and left front components respectively and are translated to the right and left front speakers as in many existing non-discrete decoding systems. A pair of passive phase-shifting lattice networks and an audio output transformer are employed to derive from the composite signals a pair of resultant signals having virtually complete separation of right back and left back components for application to the right and left back speakers respectively.

OBJECTS OF THE INVENTION It is the primary object of this invention to provide an improved system for deriving four separate audio signals from a pair of quadrature-encoded composite signals, to achieve four-channel sound rendition with substantially complete electrical and acoustical separation of the back signals in the back speakers.

It is a further object of this invention to provide such a system which maintains transmission of four distinct signals over a range of Hz-lO KHz.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawings, and in which:

FIG. 1 is a block diagram of a four-channel sound transmission system of the type to which the invention is applicable;

FIG. 2 is a block diagram of the stereo decoding system of a stereophonic receiver embodying the present invention;

FIG. 3 is a schematic circuit diagram of a portion of the receiver of FIG. 2, as used in the preferred embodiment of the invention;

FIG. 4 is a graph showing the phase angle v. frequency relationship of the two lattice networks in the preferred embodiment of FIGS. 2 and 3; and

FIGS. SA-SF display equations and vector relationships applicable to the operation of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a block diagram of a four-channel sound transmission system of the type to which the invention is applicable. The block diagram display consists of four individual input signals L L R R representing, respectively, left front, left back, right front and right back directional signals. These signals may be from a live source, or a quadraphonic tape or record disc. The individual input signals are encoded by a conventional quadrature-encoding network that develops two composite signals whose preferred form is:

E R +j 0.707 R f 0.707 L and Each of these composite signals is quadrature-encoded such that the decoding network 3 will decode the composite signals and translate the signals to the respective left front 7, right front 8, left back 9 and right back 10 sound speakers located in a conventional room orientation.

FIG. 2 is a more detailed showing of the decoder 3 (FIG. 1) embodying the invention. Units 21, 22, 23, 24 and 25 represent the preferred structure of the invention as applied to a conventional stereo sound system. The decoder comprises the right and left channel amplifiers 24 and 25 in a conventional stereo amplifier system, coupled respectively to a 90 phase-shifting circuit 21 and a 180 phase-shifting circuit 22 both of which are coupled to the transformer 23 from which resultant signals are translated to the left back and right back speakers respectively.

As shown in FIG. 3, phase-shifting networks 21 and 22 are preferably constructed as passive symmetrical lattice networks while transformer 23 comprises a secondary winding with a center tap 32. The schematic diagram in FIG. 3 displays the desired coupling of the lattice networks, the transformer and the rear speakers in the preferred embodiment of the invention. Composite signal E is coupled through the right channel amplifier 24 to the 90 phase-shifting network 21, and the second composite signal E is coupled through the left channel amplifier 25 to the 180 phase-shifting network 22. The 90 phase-shifting network 21 has its output leads 35, 36 connected to the input leads 35, 36 of the transformer 23. The 180 lattice network 22 has one output lead 37 connected to the transformers secondary center tap 32 and the other output lead 38 coupled to a common point 34 between the right-back and left back speakers 10, 9. Thus the transformer is driven in pushpull by the 90 -phase-shifted E signal and in pushpush by the 180 -phase-shifted E signal to provide both additive and subtractive matrixing.

In the preferred embodiment the first composite signal, FIG. a,

E: R, j 0.707 RB 0.707 LB is coupled through the right channel audio amplifier 24 in the conventional two-channel stereo unit 26 to the 90 symmetrical lattice network 21 which shifts the components of the composite E signal by 90. Composite signal, FIG. 5C,

E Lp 0.707 R j 0.707 L is coupled through the left channel audio amplifier to the 180 symmetrical lattice network 22 which is coupled to both the transformers secondary center tap 32 and the common point between the two rear speakers 34. Composite signals E (FIG. 5a) and E (FIG. 5c) are quadrature-encoded such that the resultant vectors A and B respectively of the back signal components of the respective composite signals are 180 out of phase. Combining the 90 -phase-shifted composite signal E (FIG. 5B) and the 180 -phase-shifted composite signal E2 (FIG. 5D) by an additive process produces a resultant signal, FIG. 5E

E4 Lp L R (3) and by a subtractive process a resultant signal, FIG. 5F

E =-L +j R 1.414 R The resultant signal E (FIG. 5F) comprising a left front, right front and right back component is coupled to the right back speaker 10 whereas the E resultant signal (FIG. 5E) comprising a left front, right front and left back component is coupled to the left back speaker 9.

As may be observed from FIGS. 5E and SF and from equations (3) and (4), the amplitudes of the respective left back and right back components in both resultant signals E and E, are enhanced by the phase shifting and matrixing action of the inventive system. Therefore, to effectively balance the acoustic output of the two rear speakers 9, 10 with that of the front speakers 7, 8, the parallel impedances of the right back and left back speakers 10, 9 are preferably matched with the design impedances of the respective and 180 lattice networks. This effectively balances the power produced at the right back and left back speakers 10, 9 with that power being produced at the right front and left front speaker systems 8, 7 and in so doing gives a balanced sound to a listener 6 (FIG. 1) who may be positioned at a spot equidistant from each of the four speaker systems.

Because the additive and subtractive matrixing cancels the right back and left back signal components in the respective E and E resultant signals, each of the respective resultant signals E and E has only one rear component, and each is coupled to its corresponding rear speaker. The substantial absence of cross-talk between the rear speakers enhances the signal separation and gives a more significant four-channel sound effect. For best results, the phase angle vs. frequency ,curve 40 (FIG. 4) of the two lattice networks should exhibit approximately a 90 phase differential 43 over a broad range 44 of audio frequencies. In the preferred embodiment, the 90 lattice network 21 and the 180 lattice network 22 are both tuned to provide an exact 90 phase differential at the center frequency 45 of the audio passband. At frequencies above and below the center frequency 45, that is, from approximately Hz to 10 kHz, the phase differential is approximately 90. An approximate 90 phase differential allows for effective cancelation of respective left back and right back signal components in the resultant E and E signals. With a 90 phase differential, there will be no left back component in the right back speaker and no right back component in the left back speaker, thus no crosstalk in each of the respective speakers; in practice it has been found that the phase differential can be held sulficiently close to 90 throughout the audio passband so that the amount of cross-talk between the rear speakers is inconsequential, and effectively complete fourchannel separation is attained.

The preferred embodiment of the invention utilizes composite signals which are quadrature-encoded such that the resultant vectors of the L and R components for each respective composite signal are out of phase, as shown in FIGS. 5A and 5B. The system of the invention is equally applicable to provide a fourchannel rendition from composite signals which are quadrature-encoded so that the resultant L and R vectors are in phase rather than in 180 counterphase.

Thus the invention provides a simple and inexpensive system for developing four separate audio signals from a pair of quadrature-encoded composite signals to effectively achieve four-channel sound rendition. The four audio signals have substantially complete electrical and acoustical separation in each of the respective speakers. 1

The invention is not limited to the particular details of construction of the embodiment depicted, and other modifications and applications may be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. A sound system for utilizing a pair of quadratureencoded composite stereo signals E and E with E comprising an RF component plus both R and L components and E comprising an LP component plus both R and L components, where L R L and R are discrete left front, right front, left back and right back quadraphonic audio signals, respectively, and where the resultant vector of the back signal components of E is either in phase or 180 out of phase with the resultant vector of the back signal components of E to provide a quadraphonic sound rendition from four sound speakers in a left front, right front, left back and right back room orientation, in which said left back speaker and said right back speaker each include a voice coil comprising a pair of input terminals with one terminal of each said pair connected to a common junction, comprising:

a pair of audio amplifiers individually translating an assigned one of said composite signals;

a passive 90 symmetrical phase-shifting lattice network coupled to one of said audio amplifiers for shifting the phase of a first of said composite signals and having a pair of output terminals;

a passive 180 symmetrical phase-shifting lattice network coupled to the other of said audio amplifiers for shifting the phase of a second of said composite signals and also having a pair of output terminals;

means for directly coupling one of said audio amplifiers to said left front sound speaker for applying said first composite signal thereto, and means for directly coupling the other audio amplifier to said right front sound speaker for applying said second composite signal thereto;

a transformer comprising a primary winding having a pair of input terminals and a secondary winding having a pair of output terminals and a reference terminal intermediate said secondary output terminals;

means for coupling the output terminals of said phase shifting network across the input terminals of said primary winding;

means for coupling the output terminals of said secondary winding across the free input terminals of said voice coils; and

means for coupling one output terminal of said phase shifting network to said reference terminal of said secondary winding and for coupling the other output terminal of said 180 phase shifting network to said common junction of said voice coils,

whereby said coupling of said phase-shifting networks to said transformer additively and subtractively combine the phase-shifted composite signals to obtain a pair of resultant signals E and E E comprising a L 'R and R component but free of any L component and E comprising a L R and L component but free of any R component, and

means for applying said resultant signal E to the right back sound speaker and said resultant signal E to the left back sound speaker.

2. A quadraphonic sound system in accordance with claim 1, wherein the E composite signal is defined as E, R +j 0.707 R 0.707 L and the E composite signal is defined as E L j 0.707 L 0.707 R 3. A quadraphonic sound system in accordance with claim 2, wherein the component values of said phaseshifting networks are such that a phase differential of approximately 90 exists between the phase shift vs. frequency curves of the 90 and 180 phase-shifting networks throughout substantially the entire audio passband, thus substantially eliminating L cross-talk in said right back speaker and R cross-talk in said left back speaker. 

1. A sound system for utilizing a pair of quadrature-encoded composite stereo signals E1 and E2, with E1 comprising an RF component plus both RB and LB components and E2 comprising an LF component plus both RB and LB components, where LF, RF, LB and RB are discrete left front, right front, left back and right back quadraphonic audio signals, respectively, and where the resultant vector of the back signal components of E1 is either in phase or 180* out of phase with the resultant vector of the back signal components of E2, to provide a quadraphonic sound rendition from four sound speakers in a left front, right front, left back and right back room orientation, in which said left back speaker and said right back speaker each include a voice coil comprising a pair of input terminals with one terminal of each said pair connected to a common junction, comprising: a pair of audio amplifiers individually translating an assigned one of said composite signals; a passive 90* symmetrical phase-shifting lattice network coupled to one of said audio amplifiers for shifting the phase of a first of said composite signals and having a pair of output terminals; a passive 180* symmetrical phase-shifting lattice network coupled to the other of said audio amplifiers for shifting the phase of a second of said composite signals and also having a pair of output terminals; means for directly coupling one of said audio amplifiers to said left front sound speaker for applying said first composite signal thereto, and means for directly coupling the other audio amplifier to said right front sound speaker for applying said second composite signal thereto; a transformer comprising a primary winding having a pair of input terminals and a secondary winding having a pair of output terminals and a reference terminal intermediate said secondary output terminals; means for coupling the output terminals of said 90* phase shifting network across the input terminals of said primary winding; means for coupling the output terminals of said secondary winding across the free input terminals of said voice coils; and means for coupling one output terminal of said 180* phase shifting network to said reference terminal of said secondary winding and for coupling the other output terminal of said 180* phase shifting network to said common junction of said voice coils, whereby said coupling of said phase-shifting networks to said transformer additively and subtractively combine the phaseshifted composite signals to obtain a pair of resultant signals E3 and E4, E3 comprising a LF, RF and RB component but free of any LB component and E4 comprising a LF, RF and LB component but free of any RB component, and means for applying said resultant signal E3 to the right back sound speaker and said resultant signal E4 to the left back sound speaker.
 2. A quadraphonic sound system in accordance with claim 1, wherein the E1 composite signal is defined as E1 RF + j 0.707 RB - 0.707 LB and the E2 composite signal is defined as E2 LF - j 0.707 LB + 0.707 RB.
 3. A quadraphonic sound system in accordance with claim 2, wherein the component values of said phase-shifting networks are Such that a phase differential of approximately 90* exists between the phase shift vs. frequency curves of the 90* and 180* phase-shifting networks throughout substantially the entire audio passband, thus substantially eliminating LB cross-talk in said right back speaker and RB cross-talk in said left back speaker. 